The invention relates to a valve for controlling liquids with a graduated step-up.
From European Patent Disclosure EP 0 477 400 A1, a valve is already known which is actuatable via a piezoelectric actuator. This known valve has an arrangement for an adaptive mechanical tolerance compensation, effective in the stroke direction, for a travel transformer of the piezoelectric actuator, in which the deflection of the piezoelectric actuator is transmitted via a hydraulic chamber.
The hydraulic chamber, which functions as a so-called hydraulic step-up means, encloses a common compensation volume between two pistons defining this chamber, of which one piston is embodied with a smaller diameter and is connected to a valve member to be triggered, and the other piston is embodied with a larger diameter and is connected to the piezoelectric actuator. By way of this compensation volume, tolerances resulting from temperature gradients in the component and possible settling effects can be compensated for without causing any change in position of the valve member to be triggered.
The hydraulic chamber is fastened between the two pistons in such a way that the actuating piston of the valve member executes a stroke that is increased by the step-up ratio of the piston diameter when the larger piston is moved a certain travel distance by the piezoelectric actuator. The valve member, piston and piezoelectric actuator are located one after the other on a common axis.
In the design of such valves, however, it must be taken into account that while the piezoelectric actuator furnishes a large force reserve as long as the actuator stroke is short, nevertheless the maximum stroke of such piezoelectric actuators is also short. By means of a hydraulic or mechanical step-up, the stroke of the actuating piston of a valve closing member can be increased relative to the actuator stroke. However, this reduces the maximum force that the actuator exerts on the valve closing member. This is a great disadvantage, especially in valves that are not force-balanced. Above all, this is true of servo valves for triggering fuel injection valves, embodied as common rail injectors, in which on the one hand a high force for opening the valve and on the other a long valve stroke is desired.
It is the object of the invention to create a valve for controlling liquids with a piezoelectric unit as its actuator system, with which both a great stroke force and a long valve stroke can be achieved.
The valve according to the invention for controlling liquid with a graduated step-up advantageously makes it possible to exert a great force on the valve closing member for a first fraction of the maximum stroke length, since the step-up ratio relative to the control piston is 1:1. Thus the valve dosing member can be opened even counter to a very high pressure. By dimensioning the sleeve surrounding the actuating piston to suit requirements, a long residual stroke length can then be overcome with lesser force.
With a graduated step-up and its simple structural design, the valve is especially well suited as a servo valve for triggering a fuel injection valve for internal combustion engines, and in particular a common rail injector, in which the servo valve must be opened counter to a high rail pressure, and a flow, predetermined by an injection nozzle, through the valve seat of the valve closing member must be achieved with a suitable valve stroke.
With the valve of the invention, the piezoelectric actuator can also be made smaller, since to execute the requisite stroke travel, the maximum actuator force is required for only a short stroke length. Since the dimensioning of the piezoelectric actuator is a significant cost factor, the production costs can additionally be lowered in this way with the valve of the invention.